Acetic anhydride is prepared in commercial quantities by the reaction of acetic acid with ketene, which itself is prepared by high temperature dehydration of acetic acid, e.g., in the presence of aluminum phosphate (AlPO.sub.4) at a temperature of 700.degree. C.
Reppe et al, U.S. Pat. No. 2,730,546, show the carbonylation of carboxylic acid esters with carbon monoxide in the presence of a cobalt halide catalyst at very high pressures to produce carboxylic acids, and their esters and anhydrides. The carbon monoxide may contain hydrogen which is disclosed to have some influence on the reaction product. In the absence of water, anhydrides are produced with excess carbon monoxide over hydrogen, e.g. no more than a 40% concentration of hydrogen. Reppe et al in Example 13 show reacting a mixture of methyl acetate, glacial acetic acid, anhydrous cobalt bromide, and an organic iodide and an organic bromide with 95% carbon monoxide and 5% hydrogen under 650 atmospheres to produce acetic acid anhydride. The acetic acid in the Reppe et al reaction mixture is used as an inert solvent for the cobalt bromide and iodide which are not well soluble in the methyl acetate, as described in Reppe et al, U.S. Pat. No. 2,789,137. The U.S. Pat. No. 2,789,137 patent additionally discloses the use of dimethyl ether as a starting material to produce acetic acid anhydride, and that the mixture of carbon monoxide and hydrogen must contain at least 60% carbon monoxide.
Paulik et al, U.S. Pat. No. 3,769,329, disclose the preparation of carboxylic acids and esters from alcohols with carbon monoxide in the presence of a rhodium and halogen catalyst to overcome the formation of undesirable by-product methane when the carbon monoxide contains an inert impurity of hydrogen. The undesirable gaseous by-product methane is disclosed to be a product of the water gas shift reaction catalyzed by cobalt.
Kuckertz, U.S. Pat. No. 4,046,807, produces acetic anhydride by reacting methyl acetate and carbon monoxide in the presence of a Group VIII metal and iodide. Hydrogen present in large quantities, 5-50% by volume, reduces soot formation and CO.sub.2, and provides an increased formation of acetic acid. Methane formation is nowhere addressed.
Rizkalla, U.S. Pat. No. 4,115,444, produces a carboxylic acid anhydride from an carboxylate ester or hydrocarbyl ether by the carbonylation of the ester with carbon monoxide in the presence of a Group VIII noble metal promoted with a metal of Group IVB, VB, VIB, or VIII, and an organo-nitrogen or organo-phosphorus compound. Rizkalla discloses a solvent of a carboxylic acid can be used which, preferably, should correspond to the anhydride being produced. The carbon monoxide should be substantially pure, but hydrogen present as an impurity is not objectionable.
Rizkalla et al in German Application No. 2610035 produce ethylidene diacetate by carbonylating dimethyl ether or methyl acetate in the presence of hydrogen and additionally disclose the production of by-product acetic anhydride and acetic acid. Acetic acid can be used as solvent or diluent to facilitate the liquid phase reaction. As such, acetic acid can be present in the range of 1 to 75 mole % of the reaction medium in the liquid phase. Formation of ethylidene diacetate requires a CO/H.sub.2 molar ratio of 2:1 to 4:1 depending on whether dimethyl ether or methyl acetate is used as the starting material. A broader range of CO/H.sub.2 of a 1:100 to 100:1 molar ratio is disclosed, and the range of 0.5:1 to 5:1 CO/H.sub.2 molar ratio is preferred. The acetic anhydride by-product depends largely on the ratio of carbon monoxide to hydrogen used. The molar ratio of by-product acetic anhydride/acetic acid can be increased by increasing the molar ratio of CO/H.sub.2. Examples are given using very high CO/H.sub.2 ratios in which significant quantities of acetic anhydride are produced, e.g. in amounts as high as 75% of the product with acetic acid present in the amount of 3%. An example producing 42% acetic anhydride using a 1 to 2 CO/H.sub.2 ratio produces substantial quantities of acetic acid. The reference nowhere discloses the amount of methane formed in the examples.
Intille, U.S. Pat. No. 4,067,900, shows in Example 10 that the hydrogenolysis of methyl acetate with a mixture of hydrogen/carbon monoxide in a very high molar ratio forms methane product. Acetic acid is used as solvent and produced as product, but quantities are not disclosed. The catalyst included an iridium component and iodide component.
The use of a mixed hydrogen and carbon monoxide atmosphere, such as in a 1/1 molar ratio, would be desirable and economically advantageous in the carbonylation reaction of carboxylic acids to form carboxylic acid anhydrides. However, the use of such a mixed gas atmosphere leads to the production of acetic acid, acetaldehyde, ethylidene diacetate, and methane as by-product compounds. The co-production of methane is of particular concern and has presented an economic obstacle to a process for forming carboxylic acid anhydrides using such a mixed hydrogen/carbon monoxide atmosphere because of the detriment to high selectivities and the expense of purging or eliminating methane from the system by conventional methods.